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A105 Stars and Galaxies

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Review: • Why was the Sun’s energy source a major mystery? • Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years • Why does the Sun shine? • The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

How does nuclear fusion occur in the Sun? • The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain • Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

Stellar Mass and Fusion • The mass of a main sequence star determines its core pressure and temperature • Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived • Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes

Star Clusters and Stellar Lives • Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations • Star clusters are particularly useful because they contain stars of different mass that were born about the same time

Evolution of a Very Low Mass Star (~0.3 solar masses) ๏ • The entire star is convective. • As hydrogen is consumed, the core shrinks and heats, the luminosity rises along the main sequence. • Since convection occurs through the whole star, all the star’s hydrogen is burned. • Leaves a helium remnant Lifetime: 300 Billion Years

Broken Thermostat • As the core contracts, H begins fusing to He in a shell around the core • Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting

End of Fusion • Fusion progresses no further in a Sun-like star because the core temperature never grows hot enough for fusion of heavier elements • Electron pressure from quantum mechanics supports the core against further gravitational contraction

The End of Solar-type Stars Main Sequence Planetary Nebula When the carbon core reaches a density that is high enough, the star blows the rest of its hydrogen into space. Red Giant White Dwarf The hot, dense, bare core is exposed! Surface temperatures as hot as 100,000 degrees The hot core heats the expelled gas and makes it glow

Planetary Nebulae • Fusion ends with a pulse that ejects the H and He into space as a planetary nebula • The core left behind becomes a “white dwarf”

Earth’s Fate • Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

Earth’s Fate • Sun’s radius will grow to near current radius of Earth’s orbit

Summary • The life stages of a Sun-like star • H fusion in core (main sequence) • H fusion in shell around contracting core (red giant) • He fusion in core • How does a Sun-like star end? • Ejection of H and He in a planetary nebula leaves behind an inert white dwarf

What about Massive Stars? • Massive stars continue to generate energy by nuclear reactions until they have converted all the hydrogen and helium in their cores into iron. • Once the core is iron, no more energy can be generated • The core collapses and the star explodes SUPERNOVA!

A “Recent” Supernova in Our Galaxy • A new star in Taurus observed by the Chinese in 1054 A.D. • Visible in the daytime • Gradually faded; gone after about two years • The Crab Nebula is a supernova remnant

The Crab Nebula Continues to Expand • The Crab Nebula is about 7000 LY away • The Nebula is about 10 LY across • Expanding at a speed of about 1,400 kilometers per second • The Crab Nebula - Then and Now • Images taken in 1973 and recently

Summary • The life stages of a high-mass star are similar to the life stages of a low-mass star • Higher masses produce higher core temperatures that enable fusion of heavier elements • A high-mass star ends when the iron core collapses, leading to a supernova

Sun-like Star Summary Main Sequence: H fuses to He in core Red Giant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell 4. Planetary Nebula leaves white dwarf behind Not to scale!

Life Stages of High-Mass Star Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell Multiple Shell Burning: Many elements fuse in shells 5. Supernova leaves neutron star behind Not to scale!

Role of Mass • A star’s mass determines its entire life story because it determines its core temperature • High-mass stars with >8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions • Sun-like stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs • Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

Abundance of Elements in the Galaxy Goals: • Know how chemical elements are created • in the Early Universe • in Stars • in Supernovae • Know how the Galaxy is enriched in chemical elements

The Origin of Elements • The process by which elements (nuclei) are created (synthesized) is called nucleosynthesis • Nucleosynthesis has occurred since the creation of the universe and will essentially go on forever • The elements created come together to form everything material we know, including us

Primordial Nucleosynthesis Hydrogen and helium were created during the Big Bang while the Universe was cooling from its initial hot, dense state. About 10% of the lithium in the Universe today was also created in the Big Bang. We’re still not sure where the rest comes from. The first stars formed from this material.